The invention relates to optical communication fibers, and more specifically to an optical communication fiber which supports at least one higher order spatial mode.
Multiniode fibers which typically support hundreds of optical modes are subject to modal dispersion. Single-mode optical fibers (SMF) which exclusively support a single optical mode, typically the LP01 spatial mode, are normally used in optical communication systems. The transmission loss in these SMFs is generally minimized for wavelengths in the range of 1300 and 1550 nm typically utilized for long-distance communications. Single mode fibers are typically utilized because they exhibit virtually no signal quality degradation as a result of modal dispersion, However, as the pulses in this range propagate through an SMF, their waveforms tend to spread because of chromatic dispersion resulting in limitations on the bit rate and the transmission distance. The total chromatic dispersion experienced is a combination of material dispersion and waveguide dispersion, which may be of opposite sign. In a conventional non-dispersion shifted SMF commonly used in the communication wavelength band of 1550 nm the total dispersion is approximately 17 ps/nmxc2x7km, which primarily reflects the material dispersion and is the maximum amount of dispersion typically experienced in transmission fibers.
Today""s communication systems demand increased bit rate and transmission distance. To accomplish these requirements, so-called dispersion shifted fibers (DSF) have been introduced. DSFs are designed to have reduced to a minimum chromatic dispersion in the typical communication wavelength band centered at 1.550 nm. However, to achieve this minimal total chromatic dispersion while still maintaining the characteristic of being a singe mode fiber, the effective area (Aeff) of a DSF must become much smaller than that of the SMF, as the waveguide dispersion is designed to counterbalance most of the material dispersion. This trade-off is well known to those skilled in the art. The smaller effective area of the DSF creates a higher intensity of optical power in the fiber for a given source, since the intensity is defined as the optical power per unit area of the fiber section. As the optical intensity in the DSF is increased due to its small effective area (Aeff), non-linear optical effects in the fiber are triggered. These effects are amplified with the square of the increased light intensity, Furthermore, these non-linear optical effects decrease the signal-to-noise ratio (S/N) which is undesirable because it can increase errors, severely limit the bit rate as well as the transmission distance.
Another technique for increasing the transmission capacity is known as wavelength division multiplexing (WDM). This technique involves using a plurality of signal wavelengths simultaneously in the fiber. This increases the overall capacity of the system as compared to a single wavelength transmission system. When WDM is used to increase transmission capacity in a DSF based system; non-linear effects known as four-wave mixing (FWM) and cross phase modulation (XWM) are generated due to the presence of the plurality of wavelengths in the fiber. As the phase matching condition between wavelengths is satisfied, FWM generation efficiency increases. For this reason, FWM is more likely to take place when the signal wavelengths are closer to the zero dispersion wavelength and the interval between signal wavelengths is smaller.
Dispersion slope is defined as the rate of change of the total chromatic dispersion of the fiber as the wavelength changes. In a conventional non-shifted SMF used in the communication wavelength band of 1550 nm it is about 0.06 ps/nm2xc2x7km. In WDM systems, without taking into account non-linear effects, a dispersion-flattened fiber would be ideal, i.e. one whose dispersion slope is as close to zero as possible. As discussed in U.S. Pat. No. 5,327,516 a certain minimum dispersion is required to prevent non-linear effects, and the absolute value of the minimum desired dispersion is approximately 2 ps/nmxc2x7km, with higher local dispersion values suppressing the FWM and XWM effects. The dispersion may be positive or negative, however compensating for positive dispersion is well known in the art by using dispersion compensating fibers which have low attenuation and high negative dispersion values (See, for example, U.S. Pat. No. 5,185,827, U.S. Pat. No. 5,261,016, and U.S. Pat. No. 5,361,319), while compensating for negative dispersion may require long lengths of fiber (See, for example, U.S. Pat. No. 4,261,639). A tradeoff may be accomplished between increasing slope, and a larger effective area (Aeff), which is exemplified in LEAF(copyright) fiber produced by Corning Incorporated, Corning, N.Y., which achieves an improved effective area (Aeff) of 72 xcexcm2 at a cost of a higher dispersion slope of between 0.07-0.10 ps/nm2xc2x7km.
Other considerations in fiber design relate to low attenuation, which is dictated by the material and concentration of dopants used, high strength, fatigue resistance and bend resistance.
Several prior art items to combat some of these problems are known to those skilled in the art, including a Large Effective Area Fiber (LEAF(copyright)) by Corning Incorporated, Corning, N.Y., and TrueWave(copyright) RS fiber by Lucent Laboratories Incorporated, Murray Hill, N.J. However both of these prior art solutions suffer from a relatively large dispersion slope, which increases total chromatic dispersion for some wavelengths, particularly in a WDM system, more than others.
U.S. Pat. No. 4,435,040 describes a W-profile single mode fiber (SMF) with minimal chromatic dispersion at two different wavelengths. However the dispersion slope is not flat, and the dispersion experienced changes in a curved fashion over the transmission waveband, which is very difficult to compensate.
U.S. Pat. No. 5,448,674 describes an optical fiber for dispersion compensation that supports the fundamental mode and the LP02 mode, but does not support the LP11 mode. It is not designed as a transmission fiber particularly due to its strong negative dispersion and sharp slope.
U.S. Pat. No. 5,781,684 describes a single mode optical waveguide with a large effective area (Aeff). This is achieved by using a segmented core profile, in which at least part of the core has a refractive index less than the clad layer. However zero dispersion is achieved in the transmission bandwidth, which as discussed above is not desirable for WDM systems. Furthermore, the dispersion slope is on the order of 0.12 to 0.16 ps/nm2xc2x7km, which over a broadband transmission spectrum is quite significant.
U.S. Pat. No. 5,835,655 describes a single mode optical waveguide fiber with a large effective area (Aeff). However zero dispersion is achieved in the transmission bandwidth, which as discussed above is not desirable for WDM systems. The effective area (Aeff) is in the order of between 70-80 xcexcm2, however, the dispersion slope is on the order of 0.08 to 0.12 ps/nm2xc2x7km, which over a broadband transmission spectrum is improved but still significant.
U.S. Pat. No. 5,878,182 describes an optical fiber designed for use in WDM systems. The absolute value of the dispersion is at least 0.8 ps/nmxc2x7km over the wavelength range 1530-1565 nm, and has a dispersion slope of approximately 0.04-0.05 ps/nm2xc2x7km. The loss is less than 0.20 db/km, and its effective area (Aeff) exceeds 50 xcexcm2, which is still significantly less than would be desired. The design is applicable to both positive and negative dispersion fibers.
The above fiber designs are all single mode fibers (SMFs), and are thus designed to support only the fundamental or LP01 mode while inhibiting all other modes.
Thus there is a need for an optical fiber with a large effective area (Aeff) for reduced nonlinear effects, minimal dispersion and dispersion slope. It would also be desired that the fiber design allow for producing fibers of both positive and negative slope, and positive and negative dispersion.
The invention relates, in one embodiment, to an optical waveguide for transmitting an optical signal having optical energy substantially in a high order spatial mode. The optical waveguide, in another embodiment, includes a few mode fiber designed to have specific transmission characteristics for supporting the single high order spatial mode, and the few mode fiber transmits the single high order spatial mode. In another embodiment, the single high order spatial mode is the LP02 spatial mode. In another embodiment, the few mode fiber supports optical energy having less than twenty spatial modes. In yet another embodiment, the few mode fiber has an effected area (Aeff) which is greater than 50 xcexcm2.
The optical waveguide, in one embodiment, has a dispersion and a dispersion slope for a given transmission bandwidth. The dispersion includes at least material dispersion, and the absolute value of the dispersion is substantially between zero and the material dispersion. In another embodiment, the dispersion of the optical waveguide is negative over the transmission bandwidth. In yet another embodiment the dispersion of the optical waveguide is positive over the transmission bandwidth. In still other embodiments, the dispersion slope of the optical waveguide is positive, negative, or nominally zero.
Another aspect of the invention includes a method for transmitting an optical signal having optical energy substantially in a single high order spatial mode. The method includes the steps of providing a few mode fiber, which supports optical energy in the single high order spatial mode. In one embodiment, the single high order spatial mode is the LP02 spatial mode In another embodiment, the few mode fiber supports an optical signal having optical energy having less than twenty spatial modes.